The present invention is a magnetoresistive (MR) sensor. More specifically, the present invention is an MR sensor having improved operational characteristics and significant processing advantages.
MR sensors or heads are used to read magnetically encoded information from a magnetic medium by detecting magnetic flux stored in the magnetic medium. During the operation of an MR sensor, a sense current is passed through the MR element of the sensor, causing a voltage drop. The magnitude of the voltage drop is a function of the resistance of the MR element. The resistance of the MR element varies in the presence of a magnetic field. Therefore, as the magnitude of the flux from a transition in a magnetic layer on a disc passing through the MR element varies, the voltage across the MR element also varies. Differences in the magnitude of the magnetic flux entering the MR sensor can be detected by monitoring the voltage across the MR element.
An MR sensor will provide an approximately linear output when the magnetization vector M of the MR element and the current density vector J of the MR element form an angle of approximately 45 degrees. An approximately linear response is achieved when these two vectors form an angle of near 45 degrees. Permalloy, a typical MR element material and an alloy of nickel and iron, will naturally tend to form a magnetization vector along its long axis when it is formed into a long narrow strip. This alignment is enhanced by a field induced anisotrophy formed during the deposition of the permalloy element. The current density vector is also typically oriented in this same direction. By forming a soft adjacent layer (SAL) near the MR element and in a parallel plane to the MR element, the magnetization vector can be rotated at 45 degrees with respect to the long axis. Therefore, if the current density vector points in the same direction as the magnetization vector pointed prior to rotation, the addition of a proper SAL will cause the output of the sensor to be nearly linearly related to the magnitude of the magnetic flux entering the MR element.
MR elements can "fracture" into multiple magnetic domains when they are exposed to an external magnetic field. To maximize the MR sensor's output and stability, it is desirable to maintain the MR element in a single domain state. Two methods for maintaining an MR element in a single domain state are hard biasing and exchange coupling. Hard biasing is accomplished by positioning a permanent magnet adjacent to the MR element. Exchange coupling occurs by depositing an antiferromagnetic layer on the MR layer so that one of the magnetic lattices of the antiferromagnetic layer couples with the magnetic lattice of the MR element layer to help preserve the single domain state of the sensor. Both hard biasing and exchange coupling can be used together.
In existing MR sensors, alignment tolerances between various thin film layers and MR sensor mask features are critical. For instance, in some MR sensor designs, the active region is defined by the placement of the contacts (or conductors). The criticality of alignment in many prior art MR sensor designs greatly increases the complexity of processing because critical geometries frequently require additional or more difficult processing steps.
There is another significant factor that adds to the complexity of prior art MR sensor designs. In magnetic sensor designs which include a separate inductive write head or transducer, the inductive write head is typically fabricated on top of the MR sensor in subsequent deposition steps. The write gap of the inductive write head must be substantially planar in order to achieve optimum results. In prior art MR sensors, planarization processing steps after fabrication of the MR sensor have been necessary to obtain a planar surface on which to deposit the write head thin film layers. As a result, many prior art MR sensor designs require additional layers and/or complex planarization techniques to achieve a planar write gap. Also, many prior art MR sensor designs have an increased separation between the reader and the writer.
In many existing MR sensors, the permanent magnet regions are placed on the sides of the MR element. In those designs, the active region of the MR element is typically defined by conductors or contacts which are placed above the MR element. However, problems arise when the active region is defined by conductors. One such problem is that the regions under the conductors near the air bearing surface absorb magnetic flux and adversely affect the reading by the MR sensor. Another problem is that the placement of the conductors can increase the complexity of processing steps aimed at planarizing the MR sensor for subsequent deposition of the inductive write sensor.